Prediction of Fracture Toughness for Open Cell Polyurethane Foams By Finite-element Micromechanical Analysis

The fracture toughness was determined for cellular polymers by micromechanical modelling using finite element analysis. In this study, mode I and mode II of fracture toughness were evaluated with a 2D-solid model using fracture analysis code FRANC2D/L. Simulation was performed for open cell polyurethane foams of different densities. Two cases were considered: constant cell length, l, and variable cell wall thickness; the former for constant cell wall thickness, t, and the latter for variable cell length. For estimation of fracture toughness the applied loads were progressively increased to the point reaching the fracture strength of the solid material (130 MPa) in an un-cracked strut in front of the crack. The estimated fracture toughness was independent on crack length, indicating that the obtained values could be considered as material property. The values of the fracture toughness of polyurethane foams are in the range of 10-3-10-1 MPa.m0.5. Lower values were obtained for mode II fracture toughness. A strong dependency of the fracture toughness on the density of the cellular material was featured by present study. The obtained results for mode I fracture toughness were compared with Gibson-Ashby micromechanical model, by which a good agreement was obtained. While, mode II fracture toughness was compared with Choi and Sankar micromechanical models. The predicted fracture toughness was finally validated with some experimental data. Another advantage of this model was to obtain a fully described the stress field in the solid struts. The stress distribution in the first un-cracked strut showed a combined stress (bending and tension) for mode I loading, while for mode II loading a pure bending appeared.